(rt'iK^xiH of Ii'(>ii-( h'ex. Ivniihilll. 355 
reuces (jf iron-ore. a mixture of hematite and magnetite, iu Breece 
Hill, and of limonite as well as a more or less ferriferous gangue 
of epigenic silver ores at Leadville. Colorado. Here transmuta- 
tion has been etfected between limestone or doUjmitie material 
and iron sulphates through aqueous infiltrations from adjacent 
intrusive porphj'ry. In the decay of the porphyry is involved 
vitriolization of pyrite. contained in this rock up to four per cent. 
The process of deposition of silver-ores as well as of ferriferous 
material, as concluded by Emmons, --was a metasomatic inter- 
change with the material in which they were deposited. '" That 
is. • -the material of w' hich thej' were composed was not a deposit 
in a pre-existing cavity in the rock" (limestone) biit -'the solu- 
tions which carried them gradually dissolved out the original 
rock material and left the ore or vein material in its place."* 
Occurrences of limonite near Hot Springs, Colorado, as de- 
scribed by >Ir. C. 31. K()lker, also atford striking examples of 
replacement of limestone.! 
lY. ReplaccuKud of Upper SUhiIku (Clindjji) Limestoiies. 
Parts of thin fossiliferous limestones of the Clinton group of 
strata are often replaced Vn' red aixl brown ferric oxides from 
extraneous sources. Tn the Appalachians of southern Pennsyl- 
vania, for example, where I have long had opportunity of closely 
oltserving the mode of occurrence of these ores, especially in 
flanks of Tussey. Dunning'sand Will's mountauis. fossil-ores, so- 
called, rarely oolitic, occupy the weathered zone of highly fos- 
siliferous beds of limestone intercalated with shales and sand- 
stones. This replacement has been wrought especially in steep 
dips by infiltrations from drainage of adjacent ferruginous strata, 
particularly of an inferior.series outcropping topographically higher 
in the Hanks of these parallel wall-like ridges. At or near water 
level or drainage level, and in topographical positions unfavor- 
able to weatlieiing action, or to sources of infiltratif>ns, replace- 
ment li:is l)een found to cease. Super-saturation as at water-level, 
and impenetration of .solutions from toi>ograpliic;d causes are 
etpudly unfavond)le for this process. 
Ill i)ortious of limestone beds lioi'deriug r:i vines down the 
mountain-side, dissolution of linu'stone sometiines has failed 
above immediate drainage level to be ;ittende<l with replacement 
*l'. 8. (ieol. Surv. xir, 1N«IJ, Pt. ',', TtH, .HO. 
tTrans. Am. Inst. 3Iin. Eng., 1.S85, \rv, '2W. 
